Hey there,
I don’t have the Slate Plus but I’ve got several of the other ones (including newer models). I’ve never had an issue accessing the UI from my android devices (I don’t have any iOS).
When you say that you “Connect to WLAN from the bottom”, what are you meaning by that? Did you connect a computer to he WLAN port then try to connect to 192.168.1? If this is the case, use the LAN port and not WLAN.
You also said you have wireguard on your mobile device. Make sure that’s not accidentally turned on when you try to connect to the slate plus (I’ve done that more than once).